WASHINGTON (CNN) -- U.S. intelligence officials warned Wednesday that the threat of al Qaeda or other terrorist groups attacking the United States was still likely -- probably in the form of a car bomb or other low-tech weapon.

But they stressed that terror groups were trying to circumvent U.S. security measures and obtain weapons of mass destruction.

"It may be only a matter of time before al Qaeda or other groups attempt to use chemical, biological, radiological or nuclear weapons. We must focus on that," CIA Director Porter Goss told the Senate Intelligence Committee.

In his first public appearance as CIA director, Goss outlined international threats to the United States, including al Qaeda, whose leaders -- Osama bin Laden and Ayman al-Zawahiri -- still remain at large.

"Their capture alone would not be enough to eliminate the terror threat to the U.S.," Goss said.

He noted that, "In the past year, aggressive measures along with key international partners have, in fact, dealt serious blows to al Qaeda and other terror organizations."

FBI Director Robert Mueller also noted al Qaeda remains intent on attacking the United States on its own soil.

"Their intent to attack us at home remains -- and their resolve to destroy America has never faltered," Mueller said in his opening statement.

"While we still assess that a mass casualty attack using relatively low-tech methods will be their most likely approach, we are concerned that they are seeking weapons of mass destruction including chemical weapons, so-called 'dirty bombs' or some type of biological agent such as anthrax."

Adm. James Loy, the acting secretary of Homeland Security, said terror groups continue to seek weapons of mass destruction (WMD) and would use them against the United States if acquired. But he added that "we are most likely to be attacked with a vehicle-borne improvised explosive device (car bomb) because that's the weapon of choice around the world."

The intelligence officials attended the open Senate hearing to discuss international threats to the United States, as well as discuss the vacant position for director of national intelligence.

"I really expected that when this hearing came, the new director of national intelligence would be here to talk about threats," said Sen. Jay Rockefeller, D-West Virginia, the vice chairman of the intelligence committee.

"Two months have now passed since the bill signing ceremony and the position of director of national intelligence remains vacant, not even a person nominated. To me, this is unacceptable."

The committee chairman, Sen. Pat Roberts, R-Kansas, said while he shared Rockefeller's frustrations, he noted the importance of choosing the right person for the position.

"It is my opinion that the administration is also awaiting the report of the independent WMD commission, part of whose job or task is to take a look at the intelligence reform bill and make some recommendations," Roberts said.

Roberts noted that, under the bill, the Bush administration had until June 17 to appoint a director of national intelligence.

U.S. officials continue to press Pakistan's government for access to nuclear scientist A.Q. Khan, Goss said.

Khan was pardoned by Pakistani President Pervez Musharraf a year ago after he confessed to supplying Iran, Libya and other countries with key nuclear parts through his sophisticated nuclear black market.

Goss said he hoped U.S. officials would be able to interview Khan to find out if his network of suppliers is still active, and how better to dismantle that network.

The CIA director also responded to a January 2002 CIA assessment of North Korea's nuclear program, which stated the communist state had produced enough plutonium for one or two nuclear weapons.

"Our assessment is they have a greater capability than that assessment," Goss said. "In other words, it has increased since then."

Goss told the committee he believed the main reason for nuclear proliferation in countries such as Iran and North Korea is not so much to stage an attack, as to keep up with their nuclear neighbors.

"I believe that having watched the pride of some countries in acquiring the world stage status of having nuclear weapons and what that has meant for nationalism and leadership is that it becomes almost a piece of the holy grail for a small country that otherwise might be victimized living in a dangerous neighborhood to have a nuclear weapon," Goss said..

"There is ... a very strong inclination by the current conservative leadership of Iran to make sure they can live up to the same levels as some of their neighboring countries, and some of those neighboring countries, indeed Pakistan comes to mind, have the bomb."